Will a 4 Ah 12 V battery be fine for a 2 A 12 V diaphragm pump + a Raspberry Pi?

Question

I've got a 4 Ah 12 V lead acid battery which currently runs a Raspberry PI. I'd like to connect a diaphragm pump to this battery as well. The pump has a max. rating of 2 A.

Assuming a fully charged battery : when the pump starts working, will this (a 4 Ah 12 V lead acid battery) be able to provide a steady voltage, or will the additional load (caused by the pump) cause undesired results? The pump will typically run for 10 - 15 minutes a day.

The Raspberry PI is apparantly notorious for being picky about the power supply.

If a 12 V 4 Ah battery is not enough; will it help if I replace the current battery with a larger one?

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Added:

Extra information:

The context of this question is that it is asked by someone who knows software development, but not electronics.

I used a RPi because that's what I had lying around and it runs Linux and Java. Easy to get started with.

The battery gets charged up every day via solar cells.

Answer 1

will this [battery] be able to provide a steady voltage

It has a "Cold Cranking Current" rating of 60A, so a pump drawing e.g. 5A starting current will be absolutely no problem.

Since the Pi needs a 5V supply, you need some kind of voltage step-down from the 12V+ battery anyway. A common step-down converter may operate on input down to about 2V above output voltage, so to about 7V for the Pi's 5V. That means that, as long as the battery's voltage does not drop below about 7V during pump start, you will be fine. And a charged 60A CCC battery in healthy condition should never drop to 7V from a 5A or even 10A current draw.

As to the run times a quick estimation:

Pump:

0.25h/day * 2A = 0.5Ah/day ~ the battery's 4Ah are enough for about 4Ah/0.5Ah/day = 8 days of pump operation.

The Raspberry Pi, if running 24/7, will however consume much more energy: Optimistically assuming maybe 3W on average, (realistically probably more like 4-6W) that'd be

3W*24h/day = 72Wh/day required for the Pi alone.

The battery holds (somewhat less usable energy than) 12V*4Ah = 48Wh

So the Pi alone will drain the battery in (much) less than 2/3 of a day (16h).

As others have stated, the Pi is probably way over the top for what you need. A small microcontroller can do many measurement, control and timing tasks as well or even better than a Pi (integrated ADC, less jitter in real-time applications, power save modes with wake-up 'cost' of a few micro- to milli-seconds,...). An AVR ATmega328 (the one also used in the Arduinos) draws about 30mA (at 5V, i.e. 0.15W) when busy. That's already down by a factor of 20-40x from an idle Pi, and, when not busy, can sleep at a fraction of that in deep power save modes.

If you need WiFi you can go for the ESP8266 or the ESP32. They will draw considerable current (couple of hundred mA's) in bursts when "busy" sending/receiving data over the air, but can also use power save modes of a few mA.

Answer 2

This is intended to complement JimmyB's excellent answer.

Solar insolation in sometimes-sunny Kristiansand in the far South of Norway is shown in the table below taken from this Gaisma page - Kristiansand, Norway

The equivalent full mean sunshine hours per day by moth are shown in the kWh/m^2/day entry. This shows that on a typical day from May to August you can expect ~= 4 to 5 SSH/day, BUT in mid Winter this falls to well under 1 SSH/day - 0.27 kWh/day/m^2 in December - if you keep the panels snow free.

Jimmy B calculates that you need 6 + 48 = 54 Wh/day for pump + Pi if the Pi runs 24/7 at fullish power. A rule of thumb for starting solar power estimates is that you get about 50% of the energy out of a battery that the panel would provide in full available sun, properly pointed, clean and snow free. In this case if you are in Kristiansand in Norway (as I'm assuming) then in Summer you will ON AVERAGE get 5 SSH (= Sunshine hours = kWh/m^2/day) from your panels. BUT in December an average of 0.27 SSH/day.
This means that to get 54 Wh/day you will need:

• In summer 54 Wh x 2 loss factor / 5 hours ~= 22 W PV panel

• In winter 54 Wh x 2 x 1/0.27 = 400 Watts of PV panel.

The winter result is for an average December day with the panel snow free and pointed at about winter optimum angle. It assumes that your panel operates well in diffuse sun conditions (this varies). If you want to have enough energy for a run of even lower sun days you'd need to add extra battery capacity and extra PV capacity.

That's a rather challenging environment for solar powering.

Wind Turbine?

A wind turbine (or snow mass turbine ? :-) ) may be better in deep winter.
A wind turbine may indeed be a serious possibility. 48 Wh/day = 2W mean = trivial for a WT IF you can keep it snow free and intact in worst case winds.
A mean wind speed of 3 m/s (11 km/h) will provide about 3 watts per m^2 of swept area at easily achievable efficiency figure of 20%. A 2m diameter 20% efficient WT will deliver 2 3 Watts in a 2 m/S (7 km/h) wind.
The blades can be relatively light weight as long as the design can tolerate high winds when necessary.

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It's quite likely that a much lower powered low cost Arduino (clones under $5 each) would do the job easily and it's likely that programming one would be easy enough for you.

If you are repeatedly discharging the battery then a low depth of discharge is desirable for long life. Full discharge daily for the battery you cited may destroy it in 100-200 discharge cycles (maybe less). Aiming at only about 20% discharge is wise or buying a "deep discharge" rated battery - which still benefit from far less than 100% DoD cycles.